I've been trying to fix this for a while and I feel like a complete idiot, so please bare with me. I figured this would be an obvious problem with an easy fix, but every time I try to find anything on this topic specifically I find threads from 2005 with broken links in them or fixes that don't work. I've read the FAQ, sticky threads, and anything else that says 'READ THIS FIRST' but I get overwhelmed trying to sort through it and keep up with various terms.

I used to be able to play flac but when I started trying to figure out how to play m4a and trying different settings (checking 'all' when selecting which file types to play in winamp/installing m4a plug ins then deleting them when they didn't work) I now can no longer play flac either. I upgraded to winamp 5.6 in hopes that these issues would be resolved, and I downloaded the Winamp Essentials pack but no luck.

I'm sure I'm missing something obvious and my ignorance is hilarious, but I'm posting here in an attempt to overcome it. We all have to start somewhere.

The reason you probably haven't gotten an answer is because we are confused. Winamp plays FLAC and M4A files natively out of the box. There is no need to download any other plugins or anything. Have you tried doing an uninstall and then reinstalling? Not just installing over the top of what you have?

I reinstalled and am now able to play flac again, but still can't (and never have) been able to play m4a. I'm not sure what more to say about it or what information would help give insight to why this is happening to me, but I can't be the only one with this problem.

i installed the Winamp essentials pack (filename: "Winamp_Essentials_6_7_8_9_10_11_12") from the winamp.com site and unfortunately, playing *.m4a files does not work. The files I want to play are downloaded via Apple's iTunes Match.
In the /winamp/system folder I can see the alac.* file. When I installed above the mentioned essentials pack, there was also no option for m4a decoder.

Listing of installed input plug-ins (in_*) and the relevent apis which they indicate that they
support to allow you to see how well the plug-in is likely to work with different Winamp builds.

Note: This is a best guess at how well the plug-ins shown should work with Winamp and until
you actually use the plug-ins with Winamp in normal use you will not otherwise know.

A * means the plug-in has support for metadata queries on a file (2.9+)
A # means the plug-in supports saving modified metadata via Winamp apis (2.9+)
A % means the plug-in supports the unified file info dialog and a '%+' shows it adds its own pages (5.5+)
A || means the plug-in supports the api_decoder used for transcoding/replaygain/etc (5.34+)

A || means the plug-in supports the api_decoder used for transcoding/replaygain/etc (5.34+)
and '(Seekable)' indicates extra support in processing of audio data via api_decoder

A {} around a symbol means the plug-in supports the unicode version of the api

'One for Nunz' found 4 installed input plug-ins in 'C:\Program Files (x86)\Winamp\Plugins'
75% (3) of the plug-ins installed provide the needed 5.x api ("winampGetExtendedFileInfo(..)
or winampGetExtendedFileInfoW(..)") to allow the formats provided by the plug-ins to work with
most of Winamp 5.x's features (more important with higher versions).

The plug-ins listed below show references to the SetWindowLongA(..) OS api. If this api is used to
subclass Winamp's main window, it can cause Winamp's display of unicode text in the taskbar to fail.
For a plug-in to be compatible it needs to use the unicode version of the api - SetWindowLongW(..).

Being listed here does not mean a plug-in will cause the issue as there are legitimate reasons for
a plug-in to be using SetWindowLongA(..) but it is a good indicator for people who know which ones
are compatible to route out plug-ins which are likely to be causing the issue. Only those plug-ins
which do not also show references to the SetWindowLongW(..) OS api have been included in this list.

If you do experience the issue then there is a way to fix the plug-in(s) if the original author is
no longer supporting or around to fix the plug-in(s) to use SetWindowLongW(..) for the subclassing
by installing the Unicode Taskbar Fix from http://nunzioweb.com/daz/unicodetaskbarfix/index.html

Okay, you need to reinstall Winamp 5.623
and this time, make sure FLAC Support and MP4 Audio Support are checkmarked
under the "Multimedia Engine > Audio" section on the Choose Components page of the installater options.